1. Field of the Invention
The present invention relates to surgical methods and devices to stabilize vertebra and promote new bone growth that will fuse adjacent stabilized vertebrae, and more particularly to coils or cages that are implanted into full-width bores into the discs of the spine and held in place with screw threads in their outside structures.
2. Description of Related Art
Degenerative disc disease accounts for more than 100,000 low back spinal fusion procedures in the United States annually, according to Columbia Colorado hospitals. The intervertebral disc is a pad of cartilage-type material situated between spinal bones. Each disc serves as a connector, spacer, and shock absorber for the spine. A soft, jelly-like center is contained by outer layers of fibrous tissue. Healthy discs help allow normal turning and bending. Trauma or injury to the spine can cause discs to tear, bulge, herniate, and even rupture. This can be quite painful, as the soft center of the disc leaks, putting pressure on the adjacent nerve roots and spinal cord.
A damaged disc can cause nerve dysfunction and debilitating pain in the back, legs and arms. Typical treatments that provide relief and allow patients to function again include back braces, medical treatment, physical therapy and surgery to remove the disc. A conventional surgical solution removes the bad disc and promotes new bone growth in the space to fuse the adjacent vertebrae together.
Such surgery is very invasive and usually requires two relatively large incisions. One of the incisions is made in the front of the body so the disc can be removed. The second incision is made in the back so connecting rods and anchor screws can be attached to the vertebrae to stabilize them long enough for the new bone to grow. But so much surgical invasion means that the recovery period can take as long as six months.
A recent invention that has been finding favor with orthopedic surgeons is the BAK INTERBODY FUSION SYSTEM by Spine-Tech Inc. (Minneapolis, Minn.). A hollow metal cylinder, or cage, about an inch long, is implanted through a small incision into the spine and into the disc space between two vertebrae. The surgical invasion is highly reduced from the previous method described and patients recover much faster. The disc is not removed whole, it is simply drilled out in two bilateral bores to receive an implant in each bore space between the adjacent vertebrae to stabilize the spine. Morselized bone is harvested from the patient and packed inside the implant. Over time, new bone will fill the inside and outside of the implants and fuse the vertebrae. The degenerated disc need not be separately removed because it is bored out wide enough in the right places and does not block the formation of new bone between two opposite sites on the adjacent vertebrae.
A clinical study on safety and effectiveness involved 947 patients that were submitted to either an anterior or posterior implantation approach. The implantation procedure requires a five-to-six inch incision in the front of the lower abdomen. Portions of disc and bone are drilled out. The BAK implants, comprising hollow threaded titanium cylinders, are screwed into the holes after bone graft is packed inside and then between the implants. Hospitalization time can be as short as three days and patients usually resume their normal activities within three months. In the study, the bone fusion rate was 90.5%, while pain was eliminated or reduced in 85.6% of cases. Functional improvement occurred in 93% of patients. There were no device-related deaths, major paralyses, device failures or deep infections. The results for the entire series showed the duration of surgery was 174 minutes, blood loss averaged 282 cc and the length of hospital stay post operation was 4.4 days. In follow up, 254 patients were evaluated after two years. The BAK device was shown to be successful in 184 patients (72%). In those patients considered successes, the spine had fused, pain was decreased, and there was no loss of muscle strength or function, e.g., the ability to sit, walk, or put on shoes. Complications were comparable to those reported in scientific literature from conventional surgery in which bone alone was used to stabilize and fuse the spine. These complications include damage to the nerve and blood vessels, infection, and the need for additional surgery to further stabilize the spine.
The clinical study was limited to the lower lumbar spine, L2-3, and L5-S1, and involved Grade 1 spondylolisthesis only. The study was limited to patients with one-level or two-level degenerative disc disease, and the researchers did not study patients with significant osteoporosis. In addition, patients with severe psychological and high functional pain were excluded from the study and patients who utilize the technology must otherwise be healthy with no cancer, heart disease or gross obesity. All surgeons in the clinical study were specially trained spinal surgeons. Such special training and specially skilled surgeons were needed because adjacent tissues, especially blood vessels and nerves, can be easily damaged during the procedure. A good deal of force must be applied to sharp, bone-cutting tools during preparation. So a guide tube for the installation tool and implant is used to provide some degree of protection of the adjacent critical tissues.
Most all of the prior art implant devices have substantial short comings, particularly in regard to how the implantation or surgical placement is to be accomplished. The Brantigan-type square cages require hammering the implant device into a prepared bed formed within the disc space. While this is a time-honored technique, it does involve the application by the surgeon of considerable force immediately adjacent to delicate nerve roots and spinal dural tissues.
Two devices recently approved by the FDA and sold commercially, e.g., by Spine-Tech and Surgical Dynamics, are threaded cylindrical devices, usually referred to as "cages". Both appear to perform better than bone graft alone. But these threaded cages have several major difficulties.
First, each implanted cage must be carefully aligned up-and-down in the spine along where the new bone is to grow. Even when properly placed, such implant devices can rotate out of alignment because of their round cross-sections.
Second, the prior art threaded cages all require a separate screw-tapping of the bone with a sharp-edged bone-cutting tool. Such requires much greater care on the part of the operating surgeon to protect nerve tissues and/or blood vessels. Both the Spine-Tech device and the Surgical Dynamics device use bulky working tubes to permit safe use of the sharp-edged bone tap. Placing the working tube safely and properly is the most difficult portion using these surgical devices.
Third, both the FDA-approved cages are made of titanium alloy metal. The titanium's greater x-ray opacity compared to bone makes post-surgical assessment of bone healing across the fusion nearly impossible.